Returning an olefin which is not reacted during the oxidation of olefins

ABSTRACT

The invention relates to a method for continuously returning an olefin which has not been reacted with hydroperoxide in order to form oxiranes during the epoxidation of olefins. Said olefin is contained in the flow of waste gas which is produced during epoxidation. Said method is characterised by the following steps: (i) compressing and cooling the flow of waste gas, (ii) separating the olefin from the flow of waste gas obtained in step (i) by distillation,(iii) epoxidating the olefin separated in (ii) with hydroperoxide.

The invention relates to a process for the continuous recirculation ofthe olefin which has not been reacted in the epoxidation of olefins bymeans of hydroperoxides to give oxiranes. In this process, the offgasstream formed in the epoxidation is compressed with cooling andseparated in a pressure distillation into an olefin-containing bottomstream and an offgas stream which is largely free of hydrocarbons. Theolefin is recirculated to the epoxidation process. The process can beemployed particularly advantageously for the recirculation of thepropene used in the preparation of propene oxide. The invention alsorelates to an apparatus by means of which the process can be carriedout.

It is known that in the epoxidation of olefins by means ofhydroperoxides to give oxiranes, the selectivity of oxirane formationdecreases significantly and the level of undesirable secondary reactionsincreases as the olefin conversion rises. To be able to achieve a highselectivity of over 95% despite this, these reactions are, especially onan industrial scale, therefore preferably carried out only to an olefinconversion of about 85 to 95%.

Isolating the unreacted olefin from the reaction process and thenrecirculating it to the oxidation process (epoxidation) is also known.Thus, a process in which a gas mixture comprising the olefin and oxygenoriginating from the decomposition reaction of the hydrogen peroxideused as hydroperoxide in the epoxidation is separated off and the olefinis absorbed from the gas mixture in a liquid absorption medium has beenproposed. In this process, a sufficient amount of an inert gas has to beadded to the oxygen to prevent formation of flammable gas compositions.In a preferred embodiment, this process is used for recovering propenefrom the reaction of propene with hydrogen peroxide to give propeneoxide. The inert gas used is preferably methane and the liquidabsorption medium used is preferably a mixture comprising isopropanoland water (EP-B 0 719 768).

In this process, the low solubility of the olefin in water-containingisopropanol has an unfavorable effect. For this reason, relatively largeamounts of solvent have to be used in order to be able to recover theolefin from the offgas stream by absorption. Furthermore, feeding afurther gas, in particular methane, in addition to the offgas stream tothe column used for the absorption is uneconomical.

It is an object of the present invention to provide an improved processfor recovering the olefin used in the epoxidation of olefins tooxiranes, by means of which it is possible to achieve more effectiverecovery of the olefin from the offgas stream than in the process of theprior art.

We have found that this object is achieved by compressing, with cooling,the offgas stream formed in the epoxidation of an olefin by means ofhydroperoxide to give the oxirane, isolating the olefin present from theoffgas stream by distillation and recirculating it to the epoxidationprocess.

The present invention accordingly provides a process for the continuousrecirculation of the olefin which has not been reacted in theepoxidation of olefins by means of hydroperoxide to give oxiranes and ispresent in the offgas stream formed during the epoxidation, whichcomprises the steps (i) to (iii)

(i) compressing and cooling the offgas stream,

(ii) separating the olefin from the offgas stream obtained in step (i)by distillation,

(iii) epoxidizing the olefin separated off in step (ii) by means ofhydroperoxide.

In the process of the present invention, it is not necessary to feed aninert gas into the separation apparatus in the olefin separation step inorder to avoid explosive mixtures. The isolation of the olefin via anabsorption plant is dispensed with. Moreover, the process of the presentinvention makes it possible to separate the olefin from the offgasstream in high yield at relatively low cost using a single distillation.Since the process can be operated continuously, it is especiallyadvantageous for industrial use.

According to the prior art, the epoxidation of olefins usinghydroperoxide can be carried out in one or more stages. Such methods andalso an industrial process are described, for example, in WO 00/07965.

To separate off the oxiranes formed in the oxidation, it is possible touse, for example, distillation columns. Here, offgas streams areobtained at the top of the columns. These offgas streams always compriseunreacted olefin and a small amount of oxygen which originates from thedecomposition reaction of the hydroperoxide used. To obtain betterregulation of the distillation, it is customary to use inert gases,preferably nitrogen. Since these are likewise taken off at the top ofthe columns, the offgas streams further comprise these gases. It istherefore no longer necessary to feed an additional gas into theabsorption plant to avoid explosive mixtures in the process of thepresent invention.

It is generally sufficient to cool the offgas stream to preferably from0 to 70° C., more preferably from 15 to 55° C., in particular from 30 to

40° C., in step (i).

These temperature ranges can, for example, be established using onlywater as coolant.

Compression can be carried out using the customary apparatuses, forexample piston compressors, diaphragm compressors, screw compressors androtary compressors.

Compression is preferably carried out not in one stage but in aplurality of stages with cooling between the individual compressionstages. This procedure has the advantage of the final compressiontemperature being able to be readily kept within the permissible range.

In a preferred embodiment of the process, compression of the offgasstream is carried out in two stages.

After compression, the offgas stream is then preferably under a pressureof from 2 to 30 bar, more preferably from 10 to 25 bar, in particularfrom 12 to 20 bar. In one embodiment of the process of the presentinvention, the offgas stream is accordingly cooled to from 0 to 70° C.and compressed to a pressure of from 2 to 30 bar in step (i).

The offgas stream which has been pre-treated by cooling and compressionis then fed to a distillation column, which may be of a conventionaltype. Such columns are, for example, rectification columns preferablyhaving from 3 to 20, more preferably from 5 to 10, theoretical plates.They can, for example, be configured as packed columns containing randomor ordered packing, as tray columns or as columns having rotatinginternals.

The compressed offgas stream is fed continuously into the column. Thefeed point can be in the middle region of the column. Depending on thephysical properties of the mixture to be separated, pressure andtemperature are selected, for instance in the ranges described above, sothat partial vaporization of the mixture to be separated occurs. Theolefin can be taken off as high-boiling component of the offgas streamat the bottom of the column and the low-boiling components, for examplethe inert gases or further volatile by-products formed in the reactionpresent in the offgas stream, can be distilled off via the top of thecolumn.

As a result of the proportion of inert gases present in the offgasstream, the separation proceeds outside the range in which explosivemixtures with oxygen can be formed. For this reason, additionalintroduction of further gases in the distillation step (ii) in order toprevent the formation of explosive mixtures becomes superfluous.

The gases originally present in the offgas stream which are obtainedtogether with the low boilers can, for example, be passed to combustion.

The olefin obtained at the bottom of the column is preferably obtainedin a purity of at least 90%. It can generally be fed without furtherpurification steps to the epoxidation by means of hydroperoxide.

In the process of the present invention, it is particularly advantageousthat the olefin can be recirculated to the epoxidation process in thatamount in which it is separated off from the offgas stream from theepoxidation. This makes possible a continuously operated process, whichis extremely economical.

The process of the present invention can be carried out particularlyadvantageously for the separation of propene from an offgas streamoriginating from the oxidation of propene to propene oxide.

As has been mentioned above, the offgas stream then comprises thepropene together with inert gases, in particular nitrogen, and a smallamount of oxygen.

In the epoxidation of propene to propene oxide, it is not necessary touse propene in high-purity form but it is instead possible to use“chemical grade” propene. Such propene contains propane, with the volumeratio of propene to propane being from about 97:3 to 95:5.

In a particularly preferred embodiment of the process of the presentinvention, the offgas stream therefore comprises propene from theepoxidation of propene to propane oxide as olefin and propane.

In this embodiment of the process of the present invention, the offgasstream is preferably cooled to from 30° C. to 40° C. and compressed to apressure of from 12 to 20 bar. A temperature of about 35° C. and apressure of about 16 bar are particularly preferred.

This compressed offgas stream is then fed into a distillation column,preferably at a point close to the uppermost theoretical plate, for thepurposes of the separation in step (ii). Choice of such a preferred feedpoint leads to the column being operated-virtually as a pure strippingcolumn. As a result, the vaporizing gas mixture within the column isenriched only relatively slightly with the high-boiling components.

This results in rapid and good isolation of the hydrocarbon-containingfraction comprising propene and propane as bottom product, while theinert gases or further volatile by-products formed in the epoxidationpresent in the offgas stream distil off as low-boiling fraction at thetop of the column. At least 93% of the hydrocarbons can be separated offfrom the offgas stream. The low-boiling fraction thus contains only lowconcentrations of hydrocarbons, preferably less than 7% by weight of thetotal components present in the low-boiling fraction.

The stream comprising the components propene and propane which isobtained via the bottom of the column can subsequently be separated intothe components propene and propane in a C₃ splitter as described, forexample, in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A22,page 214. The separation can be carried out in a column at a pressure offrom about 15 to 25 bar. The separation can also be carried out in a C₃splitter in the form of thermally coupled columns. These can, forexample, be operated at a pressure of about 15 or 25 bar. The propene isobtained at the top while the propane is obtained at the bottom.

The propene can then be returned to the epoxidation by means ofhydroperoxide. Propane can be used as energy source for steamgeneration.

Examples of olefins which can be separated off by means of the processof the present invention from offgas streams which are formed in theepoxidation of these olefins by means of hydroperoxide to give thecorresponding oxiranes are the following compounds:

ethene, propene, 1-butene, 2-butene, isobutene, butadiene, pentenes,piperylene, hexenes, hexadienes, heptenes, octenes, diisobutene,trimethylpentene, nonenes, dodecene, tridecene, tetradecane to eicosene,tripropene and tetrapropene, polybutadienes, polyisobutenes, isoprene,terpenes, geraniol, linalool, linalyl acetate, methylenecyclopropane,cyclopentene, cyclohexene, norbornene, cycloheptene, vinylcyclohexane,vinyloxirane, vinylcyclohexene, styrene, cyclooctene, cyclooctadiene,vinylnorbornene, indene, tetrahydroindene, methylstyrene,dicyclopentadiene, divinylbenzene, cyclododecene, cyclododecatriene,stilbene, diphenylbutadiene, vitamin A, beta-carotene, vinylidenefluoride, allyl halides, crotyl chloride, methallyl chloride,dichlorobutene, allyl alcohol, methallyl alcohol, butenols, butenediols,cyclopentenediols, pentenols, octadienols, tridecenols, unsaturatedsteroids, ethoxyethene, isoeugenol, anethole, unsaturated carboxylicacids such as acrylic acid, methacrylic acid, crotonic acid, maleicacid, vinyl acetic acid, unsaturated fatty acids such as oleic acid,linoleic acid, palmitic acid, naturally occurring fats and oils.

Preference is given to using olefins which are gaseous under normalconditions or have a boiling point below 150° C. in the process of thepresent invention. These are preferably olefins having from 2 to 8carbon atoms. Particular preference is given to using ethene, propeneand butene. The use of propene is most preferred.

Hydroperoxides which can be used for the epoxidation are all thehydroperoxides which are known from the prior art and are suitable forreaction with the olefin. Examples of such hydroperoxides are tert-butylhydroperoxide and ethylbenzene hydroperoxide. Hydrogen peroxide can alsobe used as hydroperoxide, for example as an aqueous solution.

The offgas streams can also originate from oxidation processes in whichthe reaction of the olefin with the hydroperoxide is catalyzed, forexample by means of heterogeneous catalysts.

In a particularly preferred embodiment of the process of the presentinvention, propene is separated off from an offgas stream obtained inthe heterogeneously catalyzed epoxidation of propene by means ofhydrogen peroxide to give propene oxide.

The invention also provides an apparatus for carrying out the process ofthe present invention, which comprises at least one reactor forpreparing propene oxide, at least one apparatus for compressing theoffgas stream, at least one distillation column for separating propeneand propane from the offgas stream and a C₃ splitter for separatingpropene and propane.

The flow diagram shown in the figure indicates how propene can berecovered from the epoxidation (oxidation) of propene to propene oxideby the process of the present invention, with compression being carriedout in two stages with intermediate cooling.

LIST OF REFERENCE NUMERALS FOR THE FIGURE

-   A offgas from epoxidation-   V compressor-   W heat exchanger-   I inert gases (n₂), O₂-   B distillation column-   C C3 splitter-   P propane and possibly further high boilers-   c3″ “chemical grade” propene

1. A process for the continuous recirculation of an olefin which has notbeen reacted in the epoxidation of olefins by means of hydroperoxide togive oxiranes and is present in the offgas stream formed during theepoxidation, which comprises the steps (i) to (iii) (i) compressing andcooling the offgas stream, (ii) separating the olefin from the offgasstream obtained in step (i) by distillation, (iii) epoxidizing theolefin separated off in step (ii) by means of hydroperoxide.
 2. Aprocess as claimed in claim 1, wherein, in step (i), the offgas streamis compressed to a pressure of from 2 to 30 bar and cooled to from 0 to70° C.
 3. A process as claimed in claim 1 or 2, wherein compressionoccurs in at least two stages in step (i).
 4. A process as claimed inany of claims 1 to 3, wherein the offgas stream comprises propene andpropane.
 5. A process as claimed in claim 4, wherein, in step (i), theoffgas stream is cooled to from 30 to 40° C. and compressed to apressure of from 12 to 20 bar.
 6. A process as claimed in claim 5,wherein the mixture of propene and propane obtained in the bottoms fromthe column after the distillation in step (ii) is separated into propeneand propane in a C₃ splitter.
 7. An apparatus for carrying out a processaccording to claim 6, which comprises at least one reactor for preparingpropene oxide, at least one apparatus for compressing the offgas stream,at least one distillation column for separating propene and propane fromthe offgas stream and a C₃ splitter for separating propene and propane.